No2 o3no3 o2 no2 no3n2o5

These gas-phase processes normally are reversible and do not remove much N02 from the air, but in the presence of high levels of liquid droplets, a conversion of N205 to nitric acid occurs instead:

H2SOi droplets

By this mechanism, much of the N02 that normally would be available to tie up chlorine monoxide as the nitrate C10N02 becomes unavailable for this purpose; hence a greater proportion of the chlorine atoms occur in the catalytically active form and destroy ozone. It should be realized, however, that even in the absence of particles, some N02 is converted to nitric acid as a result of its reaction with the hydroxyl radical. This nitric acid eventually

(continued on p. 76)

BOX 2-1

The Chemistry Behind Mid-Latitude Decreases in Stratospheric Ozone (continued)

undergoes photochemical decomposition in daylight hours to reverse this reaction and to produce species that are catalytically active in ozone destruction.

In the mid-latitude lower stratosphere, the most important catalytic ozone destruction reactions involving halogens employ Mechanism II, with X being atomic chlorine or bromine and X' being the hydroxyl radical:

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